Fork-lift trucks have been provided in the prior art with a pair of driven wheels and a driven, steerable third wheel which not only facilitates steering but also gives considerably more traction when needed such as in mud, soft soil or the like. These wheels are of the same size and are driven by hydraulic motors associated with the respective wheels. The steerable wheel is mounted for pivoting movement about a vertical axis and a rack and pinion arrangement has been used in the prior art for controlling the pivoting of the wheel support about the vertical axis. With tractors having the driven three wheel arrangement all of the wheels are driven at the same time but the drive for the third wheel is not always needed when conditions of the terrain are good which results in an unnecessary expenditure of fuel.